As a student of Psychology at University of Massachusetts, I was once assigned a research project on altered states of consciousness. Having already experienced pscilocybin mushrooms 60 separate times, as well as LSD, MDMA, MDA, and Salvia on a few occasions, I was excited to be able to discuss such a topic in purely objective terms.
First, you can absolutely hallucinate off marijuana. Marijuana itself is a mild hallucinogen and smoking an extreme amount will induce hallucinations.
There are 3 categories of Hallucinogens.
1-Psychedelics.
Mushrooms, marijuana, and LSD all fall under this category. The human brain has a filtering system for perception, allowing us to perceive only a limited amount of the onformation received by our senses. Psychedelics essentially turn off this filter, flooding our brains with sensory information. Seeing as our brains are constantly working to organize sensory information to construct our perceived reality, the increase in information requires the brain to organize more data. This results in an altered state of consciousness.
Interesting to note is that pscilocybin (mushrooms) has a molecular structure very similar to the neurotransmitter serotonin and essentially acts as a neurotransmitter upon ingestion. It actually binds to serotonin and alters the information carried on the neurotransmitter, effectively hijacking the serotonin.

2-Dissociatives
Salvia best represents the effects of these hallucinogens. The brain experiences sensory deprivation and sensory separation, leading the brain to essentially "fill in the blanks" when it comes to information relating to perception of reality. There are ways of inducing sensory deprivation without the use of drugs, such as meditation, sensory deprivation tanks, and lucid dreaming.

3- Deliriants
Datira seeds and other deliriants shut off your acetylcholine neurotransmitter which is known to work in relation to memory, as Alzheimers parients all show a significant depletion of acetylcholine. Trip reports are never good, for the effects it has on this vital neurotransmitter can lead to one perceiving their mirror reflection as a stranger.

DMT is hard to categorize because it is all pervasive in its effects. It works as a psychedelic and a dissociate. Its chemical structure is also of extreme similarity to that of serotonin and pscilocybin. Due to it existing within the brain, DMT is theorized by some to be a neurotransmitter that remains inactive, firing only at birth and death. However, DMT's specific function is still a mystery and no clear explanation can be given on its profound hallucinations